This invention pertains to Fourier transform nuclear resonance spectroscopy and, in particular, relates to the removal or compensation of acoustic ringing phenomena from pulse excited resonance data.
The modern practice of NMR spectroscopy employs pulsed excitation of the resonating systems followed by acquisition of the time domain response, that is to say, a waveform. The information contained in the waveform yields a frequency spectrum upon appropriate Fourier transformation. A number of parasitic effects accompany the practice of FT-NMR and the resulting artifacts in the spectrum may be minimized by appropriate measures directed to the cause, or in many instances the artifact may be cancelled by exploiting the difference in coherence properties between the nuclear resonance signal and the artifact.
It is known that high frequency currents in conductors situated in static magnetic fields can give rise to standing ultrasonic waves which through a reverse mechanism produce a resonant-like signal. The parasitic effect has often been observed and is discussed at length by Fukushima et al, Journal of Magnetic Resonance, Vol. 33, pages 199-203 (1979) who also discuss measures for minimizing the effect by choice of materials and equipment design configuration.
Acoustic ringing is a broad resonance-like phenomenon reflecting the strong coupling of the high frequency current with a crystal lattice in the static field. The resulting standing wave is therefore characterized by rather short decay times. Nevertheless, the amplitude is large and short duration NMR signals may be completely lost in comparison with this transient.
In the prior art it has been recognized that short-lived resonances may be removed from spectra by the simple expedient of interposing a delay between the excitation and detection of resonance. Relatively long NMR signals are observed without difficulty in the presence of the short acoustic ringing by delaying the acquisition for a suitable period to permit the acoustic ringing to attenuate. Unfortunately this strategy will introduce a frequency-dependent phase shift for which compensation will ultimately be required and separately the signal-to-noise factor will be reduced by the decay of signal strength during the delay period.
In a more sophisticated elaboration of the conquer-by-delay approach, a spin echo measurement has been used to substantially preserve the resonance signal during the decay of the instrumental ringing. The phase shift introduced by the simple delay and acquire method is eliminated by the spin echo measurement, but a signal attenuation factor, e.sup.-2 .tau./T.sub.2 must be sustained. Moreover, spin echo measurements introduce acoustic ringing with the inversion pulse as well as with the observe pulse and the choice of refocussing time is necessarily long enough to permit the inversion pulse ringing to attenuate.